I think in part it has to do with other classes having access to none potion/consumable forms for DR. The warrior class is unique that is has no standard class DR i.e sanc, trance, etc, and cannot use one or more of the following scrolls, talismans, or wands.

Except for assassins (and they get trance for DR) it feels like other classes all have easier means of prepping for DR then trying to farm limited shield/aura preps.